Chemical processes for the production of fluorocarbons typically produce a number of impurities along with the desired product or products, which are then isolated by fractional distillation. The knowledge that any azeotropes exist, along with their composition and formation conditions, are vitally important for the efficient operation of these isolation processes. Such azeotropic compositions may also be valuable as products.
The fluorocarbon compositions described in this invention can occur as part of certain manufacturing processes and are believed to be useful as cleaning agents, expansion agents for polyolefins and polyurethanes, refrigerants, aerosol propellants, heat transfer media, gaseous dielectrics, fire extinguishing agents, powercycle working fluids, polymerization media, particulate working fluids, carrier fluids, buffing abrasive agents and displacement drying fluids.
One of the important uses of fluorocarbons is as a cleaning agent or solvent to clean, for example, electronic circuit boards. Electronic components are soldered to circuit boards by coating the entire circuit side of the of the board with flux and thereafter passing the flux-coated board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder fusion, but leaves residues on the circuit boards that must be removed with a cleaning agent. Preferably, cleaning agents should have a low boiling point, non-flammability, low toxicity, and high solvency power so that flux and flux-residues can be removed without damaging the substrate being cleaned. Further, it is desirable that cleaning agents that include a fluorocarbon be azeotropic or azeotrope-like so that they do not tend to fractionate upon boiling or evaporation. If the cleaning agent were not azeotropic or azeotrope-like, the more volatile components of the cleaning agent would preferentially evaporate, and the cleaning agent could become flammable or have less desirable solvency properties, such as lower flux solvency and lower inertness toward the electrical components being cleaned. The azeotropic property is also desirable in vapor degreasing operations because the cleaning agent is generally redistilled and reused for final rinse cleaning.
Fluorocarbons may also be used as refrigerants. In refrigeration applications, a refrigerant is often lost during operation through leaks in shaft seals, hose connections, solder joints and broken lines. In addition, the refrigerant may be released to the atmosphere during maintenance operations on refrigeration equipment. Accordingly it is desirable to use a single fluorocarbon or an azeotrope or azeotrope-like composition that includes one or more fluorocarbons as a refrigerant. Nonazeotropic compositions have the disadvantage of changing composition, or fractionating, when a portion of the refrigerant charge is leaked or discharged to the atmosphere. Refrigeration equipment operation could be adversely affected due to the change in composition and vapor pressure that results from fractionation.
Azeotropic or azeotrope-like compositions of fluorocarbons are also useful as blowing agents in the manufacture of close-cell polyurethane, phenolic and thermoplastic foams. Insulating foams require blowing agents not only to foam the polymer, but more importantly to utilize the low vapor thermal conductivity of the blowing agents, which is an important characteristic for insulation value.
Aerosol products employ both single component fluorocarbons and azeotropic or azeotrope-like compositions of fluorocarbons as propellant vapor pressure attenuators in aerosol systems. Azeotropic or azeotrope-like compositions, with their substantially constant compositions and vapor pressures, are useful as solvents and propellants in aerosols.
Azeotropic or azeotrope-like compositions that include fluorocarbons are also useful as heat transfer media, gaseous dielectrics, fire extinguishing agents, power cycle working fluids such as for heat pumps, inert media for polymerization reactions, fluids for removing particulates from metal surfaces, and as carrier fluids that may be used, for example, to place a fine film of lubricants on metal parts.
Azeotropic or azeotrope-like compositions that include fluorocarbons are further useful as buffing abrasive detergents to remove buffing abrasive compounds from polished surfaces such as metal, as displacement drying agents for removing water such as from jewelry or metal parts, as resist-developers in conventional circuit manufacturing techniques employing chlorine-type developing agents, and as strippers for photoresists when used with, for example, a chlorohydrocarbon such as 1,1,1-trichloroethane or trichloroethylene.
Some of the fluorocarbons that are currently used in these applications have been linked to depletion of the earth's ozone layer and to global warming. What is needed, therefore, are azeotropic or azeotrope-like substitutes for such fluorocarbons that have low ozone depletion potentials and low global warming potentials.
Production of certain fluorocarbons can result in the inadvertant production of azeotropes, as for example when hydrogenating an unsaturated perfluorinated compound, or partially fluorinating a chlorofluorocarbon species. The recovery of azeotropes or azeotrope-like mixtures from the waste streams of existing chemical manufacturing processes makes it feasible to turn a waste product into a useful one, benefitting the environment.
U.S. Pat. No. 3,173,872 discloses that mixtures of difluoromethane (HFC-32) and perfluoropropane (PFC-218) have a nearly constant boiling point through the range of perfluoropropane content ranging from about 10 to 62 mol %.
From the open chemical literature (Horseley, L. H., "Azeotropic Data-III", Advances in Chemistry Series #116, American Chemical Society, Washington DC, (1973)), it is known that minimum boiling azeotropes are formed by binary mixtures of: 1-chloro-1,1,2,2-tetrafluoroethane (HCFC-124a) and perfluorocyclobutane (PFC-C318), chlorodifluoromethane (HCFC-22) and perfluoropropane (PFC-218), and chlorodifluoromethane and hexafluoropropylene (PFC-1216).